FU money is an extremely relative term. It depends on who you are saying FU to and on what terms, how easily you can make money, what the normal length of contract in that industry is, whether it's normally time-limited or ongoing, etc.

If you live (relatively speaking) from hand to mouth, and you walk into a negotiation for a small amount of work, it doesn't take much money to be able to say No and mean it; enough money to last a week may be FU money.